I don't see that any more description was necessary to answer the basic question:
Q: Going from N40 to N42 with the same magnet geometry, what's the effect on an alternator and do I have to make an adjustment elsewhere to compensate?
A: They are stronger by a factor of (x), which is enough that you might want to take advantage of the extra strength (which produces a corresponding increase in voltage), winding about 1/x turns of wire of a correspondingly increased wire cross section (adjusting the gauge and/or in-hand count, getting the closest you can fit to a factor of X) to instead maintain the same voltage and increase the current by a factor of your cross-section increase. Alternatively, you can throw away the extra strength by widening the gap and leaving the coils as they are, giving you a larger tuning range. Later, if it turns out that your winds are lower than you planned for, you can reduce the gap to bring back some of the extra field strength and use it to obtain a lower cutin speed.
Why do you need to know the mill's size, color scheme, furling scheme, blade count, phase count, pole count, turns count, wire size, blade profile, or which way it rotates to give an answer of the above form?